Sandia National Laboratories is investigating advanced Brayton cycles using supercritical working fluids for use with solar, nuclear or fossil heat sources. The focus of this work has been on the supercritical CO 2 cycle (S-CO2) which has the potential for high efficiency in the temperature range of interest for these heat sources, and is also very compact, with the potential for lower capital costs. The first step in the development of these advanced cycles was the construction of a small scale Brayton cycle loop, funded by the Laboratory Directed Research & Development program, to study the key issue of compression near the critical point of CO 2 . This document outlines the design of the small scale loop, describes the major components, presents models of system performance, including losses, leakage, windage, compressor performance, and flow map predictions, and finally describes the experimental results that have been generated. 4

The two cycles selected for final consideration are the UT-3 cycle and the Sulfur-Iodine (S-I) cycle. The UT-3 cycle was invented at the University of Tokyo and much of the early development was done there. This cycle has been studied extensively by the Japan Atomic Energy Research Institute (JAERI). After considering several different flowsheets making use of the UT-3 cycle, JAERI selected the so-called Adiabatic UT-3 process for further development. The predicted efficiency of the Adiabatic UT-3 process varies between 35% and 40% depending upon the efficiency of membrane separators which are under development. A 10% overall efficiency increase is projected if electricity is co-generated along with the hydrogen.

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